FR2782632A1 - EXPANSIBLE INTERSOMATIC FUSION CAGE - Google Patents

Abstract

An expandable interbody fusion device in one embodiment includes a cylindrical body (10) defining a hollow interior (17) for receiving bone graft or bone substitute material. The body (10) is divided into a number of branches (24, 26, 40 and 41) connected to one another at a fixed end (20) and separated at an expandable end (18). The expandable cage may be inserted in its substantially cylindrical form and may be expanded by movement of an expansion member (50) to establish lordosis of the spine. The present invention provides an expansion member (50) that interacts with the interior surfaces of the device to maintain the cage in the expanded condition and provide a large internal chamber (17) for receiving bone in-growth material. Methods for insertion of the fusion device are also disclosed.

Description

BACKGROUND OF THE INVENTION

  The present invention relates to an implantable device for promoting the

  fusion of two adjacent bone structures and a method of insertion thereof.

  More particularly, the invention relates to an expandable fusion cage which can be inserted, in a reduced size configuration, into an intervertebral space and be expanded after insertion to obtain a desired size. While the device according to the present invention may have applications in other regions of the body, the present invention is preferably used for the fusion of

two vertebral bodies.

  There are a significant number of attempts to develop an exceptional intradiscal implant which could be used to replace a damaged disc and yet maintain the stability of the disc space between adjacent vertebrae, at least until the complete completion of arthrodesis. These "interbody fusion devices" have taken many forms. For example, one of the

  The most important structures are in the form of a cylindrical implant.

  These types of implants are described in US Pat. Nos. 4,501,269 in the name of Bagby; n 4,878,915 in the name of Brantigan, n 4,961,740 and 5,055,104 in the name of Ray and 5,015,247 in the name of Michelson. In cylindrical implants, the outer portion of the cylinder can be threaded to facilitate insertion of the interbody fusion device, as shown in the patents named Ray, Brantigan and Michelson. Alternatively, some fusion implants are provided to be deformed in the interdisc space. This type of device is

  represented by the patent in the name of Brantigan.

  Interbody fusion devices can be broadly divided into two basic categories, namely solid implants and implants that are intended to allow bone growth inside. The solid implants are represented by US Pat. Nos. 4,879,915, 4,743,256, 4,349,921 and 4,714,469. The other aforementioned patents include an aspect which allows bone growth through the implant. It has been found that the devices, within which natural bone growth is promoted, achieve faster and more stable ahrodesis. The device described in the patent in the name of Michelson is representative of this type of hollow implant which is typically filled with a substance inducing bone growth to promote bone growth in and through the device. This implant includes a plurality of circular openings which communicate with the hollow interior of the implant, thereby providing a passage for tissue growth between the vertebral plates and the bone growth material within the implant. In the preparation of the interdiscal space, the plates are reduced preferably to the bleeding bone to facilitate tissue growth inside. During the fusion, the metallic structure provided by the Michelson implant helps to maintain the solidity and stability of the segment ensuring the movement that is to merge. In addition, as soon as the arthrodesis appears, the implant itself serves as a sort of element

  anchor for solid bone mass.

  A problem which the devices of the prior art do not address concerns the maintenance and restoration of the normal anatomy of the fused vertebral segment. Naturally, once the disc is removed, the normal cyphotic or lordotic curvature of the spine is removed. With the devices of the prior art, the need to restore this curvature is neglected. For example, the adjacent vertebral bodies can be bored with a cylindrical reamer which adapts the particular implant. In some cases, the normal curvature is established before reaming and then the implant is inserted. However, this excessive bore in the posterior portion is generally not well accepted due to the removal of the bony portion of the load-bearing vertebrae and because it is typically difficult to achieve bore in the posterior portion of the lower lumbar segment , where lordosis is most important. In most cases using implants of this type, no effort is made to restore the lordotic curvature, so that the cylindrical implant is likely to cause a cyphotic deformity when the vertebra is placed around the implant. This phenomenon can often lead to restorative surgeries because

  that the spine is unbalanced.

  In each of the aforementioned patents, the cross section of the implant is substantially constant over its entire length and is typically in the form of a straight circular cylinder. Other implants have been developed for interbody fusion that do not have a constant cross section. For example, US Pat. No. 4,714,469 in the name of MacKenna describes a hemispherical implant having elongated protuberances which protrude into the vertebral end plate. In addition, US Patent No. 5,669,909 to Zdeblick et al. describes a truncated cone implant capable of being received by screwing into the intervertebral space. However, these devices require an opening at least as large as the largest segment of the device. The need for a relatively large opening may limit the use of such devices, particularly where access to the spine is limited due to vessels and structures

neurological obstruction.

  Other implants have also been developed which have the ability to adjust the size of the implant after insertion. U.S. Patents 5,665,122 in the name of Kambin, 5,554,191 in the name of Lahille et al. and 5,653,763 to Errico et al., describe implants which provide at least some degree of implant height adjustment to restore lordosis. However, these implants do not allow the device to be inserted easily and securely in an interdiscal space, and the internal expansion mechanism limits the ability to fill the interior.

  a large amount of material that promotes bone growth.

  Given these limitations of the devices of the prior art, there remains a need for an interbody fusion cage capable of stabilizing the spine in a manner comparable to the structures of interbody fusion implant currently used, and at the same time capable of provide a mechanism to restore normal lordosis of the spine. After expansion, the implant must have an internal cavity suitable for receiving a graft

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  bone or a bone substitute to promote bone growth through

the extended implant.

SUMMARY OF THE INVENTION

  To meet the needs not yet resolved by the devices of the prior art, the present invention provides an expandable (extensible) fusion cage capable of being inserted between a pair of vertebral bodies to restore the normal angular relationship between adjacent vertebrae. In particular, a device according to the present invention comprises a body comprising an external surface engaging with the bone and an internal surface defining an internal chamber and a retaining mechanism. The body has a fixed portion and an expandable (expandable) portion. The expandable portion is divided into at least a first branch and a second branch, interconnected at the level of the fixed portion. The first branch is movable relative to the second branch in the expandable portion to expand the general size of the fusion cage. An expansion element is dimensioned to be at least partially received inside the internal chamber and has an external surface capable of engaging with the retaining mechanism. The movement of the extension element (expansion) inside the internal chamber pushes the first branch to move it relative to the second branch thus expanding the expandable portion. The expansion element is held in position by engagement with the release mechanism

body restraint.

  In another aspect of the invention, the extensible interbody cage includes a tubular body having an outer surface engaging with the bone and an inner surface defining an internal chamber. The interior surface further defines an inclined surface and a retaining mechanism. The body defines a fixed portion and a movable portion, the inclined surface is arranged near the movable portion. In this aspect of the invention, an extension member is included which has an outer surface configured to engage with the inclined surface and the retaining portion. The movement of the extension member against the inclined surface moves the movable portion to extend the cage. The extension member is held in position by engagement with the retainer. In another embodiment, the invention provides an insertion tool which cooperates with an extensible fusion implant. The insertion tool is operable to insert the implant and move the extension element to expand the cage. A cage according to the present invention is used with an insertion tool comprising an outer sleeve capable of engaging with the body of the cage to transmit a compressive force to the cage. The insertion tool further includes an inner axis movably arranged inside the outer sleeve. The internal axis is dimensioned to be inserted in at least a portion of the internal chamber of the cage. The inner shaft has a distal end configured to retractably engage the extension member, such that movement of the inner shaft relative to the outer sleeve moves the member

extension in relation to the cage.

  In another aspect of the invention, methods are provided for inserting an expandable interbody device. In a first aspect, a device according to the present invention is inserted into the interdisc space. The movable portion of the device is positioned to be adjacent to a portion of the vertebra requiring additional spacing. The extension element is then moved inside the internal chamber and against the movable portion to expand the cage to the desired height. Extension element can be engaged

  with the retaining mechanism to limit the movement of the extension member.

  A first object of the present invention is to provide an implant which has a reduced insertion configuration and which is extensible from the

  insertion configuration up to a larger configuration.

  Another object of the present invention is to provide an expandable implant which has a substantially unobstructed internal chamber for receiving

  material promoting bone growth.

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  Yet another object of the present invention is to provide a fusion cage configured for easy insertion and expandable to a larger size.

to establish lordosis.

  Yet another object of the present invention is to provide an improved method for inserting a fusion cage and restoring lordosis. Objects and associated advantages of the present invention will become apparent from the

following description.

BRIEF DESCRIPTION OF THE DRAWINGS

  Figure 1 is a top view of an expandable cage according to a

  O10 first embodiment of the present invention.

  FIG. 2a is a view in longitudinal section of the device according to 2- 2 of

Figure 1.

  Figure 2b is the device of Figure 2a in which is included a wedge extension element according to a first embodiment of the

present invention.

  Figure 3 is a perspective view and partial section of the cage

  extendable of Figure 1 without external thread.

  Figure 4 is a perspective view of the corner extension member of the

Figure 2b.

  Figure 5 is an end view of the corner extension member of the

figure 4.

  Figure 6 is an elevational view of an insertion tool according to a

  first embodiment of the present invention.

  Figure 7a is a view in sagittal plane in side elevation and partial section of the extensible cage of Figure 2b inserted between two adjacent vertebrae in an insertion configuration according to a first mode of

  realization of the present invention.

  Figure 7b is the cage of Figure 7a shown in a position

  expanded in accordance with a first embodiment of the present invention.

  FIG. 8 is a top view of a variant of the embodiment of

the extendable cage of figure 1.

  Figure 9 is a sectional view of the extendable cage according to 9-9 of the

figure 8.

  Figure 10 is a perspective view and partial section of the cage

  extensible from Figure 8 without external thread.

  Figure 11 is a top view of another embodiment of a

  extendable cage according to the present invention.

  Figure 12 is a longitudinal sectional view of the extendable cage of the

figure 11.

  Figure 13 is a perspective view and partial section of the cage

  extendable in Figure 11, without external thread.

  Figure 14 is a longitudinal sectional view of another embodiment of

  realization of the present invention.

  Figure 15 is a longitudinal elevational view in partial section

  yet another embodiment of the present invention.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS

  In order to better understand the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific terminology will be used to describe them. It should nevertheless be understood that the scope of the invention is in no way limited to these, additional changes and modifications of the devices illustrated, and additional applications of the principles of the invention as illustrated here can normally be envisaged by the skilled person concerned by the

field of the invention.

  Referring now to Figures 1 to 5, there is shown a first embodiment of an expandable (or expandable) cage 10 according to the present invention. In this embodiment of the invention, the extendable cage comprises a cylindrical external surface 11 defining an external thread 12 (not shown in FIG. 3) capable of engaging with two adjacent vertebrae (see FIGS. 7a and 7b) and advancing the cage in the interdiscal space when the cage 10 is rotated about the longitudinal axis 13. As shown more clearly in FIG. 3, the extendable cage 10 is hollow with an internal surface 21 defining an internal cavity 17. The extendable cage 10 includes a bone growth window 16 formed in the extendable branch 24 and a

  identical bone growth window 19 formed in the extensible branch 26.

  These extendable windows are capable of allowing communication between the vertebral bodies (FIG. 7) and the internal chamber 17. In most applications, the material promoting bone growth will be placed inside the internal chamber 17 of the cage. expandable 10 to promote bone growth in the

  fusion device 10 and through it.

  As shown in the accompanying drawings, the extendable cage 10 is preferably composed of four separate branches, each being separated at the extendable end 18 by a channel extending longitudinally from the extendable end 18 in the direction of the fixed end 20. Referring now to FIG. 2a, a first extensible branch 24 is separated from the first fixed branch 40 by a channel 14. Similarly, a second extensible branch 26 is separated from the first fixed branch 40 by a channel 22. Each of the channels 14 and 22 extends from the extendable end 18 in the direction of the fixed end 20. The channels terminate in a rounded opening of slightly larger diameter which preferably acts as a hinge for the extension of the device for concentrating the stresses and deformations in the vicinity of the fixed end 20. Similarly, as shown nté in Figure 3, the second fixed branch 41 is separated from the first extendable branch 24 by a channel 15 and the second extendable branch 26 by a similar channel (shown). Thus, the extendable cage 10 is formed of four branches, each being separated from the other at the extendable end 18 by channels extending from the outer surface 11 to the internal chamber 17. The branches are connected to the fixed end 20 by a connection zone 44 so that each of the branches can move substantially independently of each of the others at the extendable end 18 while remaining connected to the device by the connection zone 44. Although the present embodiment uses integral branches, it is intended that separate components can

  be connected to form the extendable cage without departing from the invention.

  While four separate branches are shown in a preferred embodiment, provision is made to be able to use a greater or lesser number of branches without departing from the spirit or the scope of the invention. In addition, although in a preferred embodiment the channels extend from the outer surface 11 to the internal chamber 17, it is intended that the channels cannot extend to the internal chamber. Such a channel can be formed by an overlapping interface between two adjacent branches without creating an opening for bone growth inside the internal chamber 17. In addition, although channels are shown as being preformed in the extendable cage , provision is made to be able to form the channels when the implant is expanded. By way of example, and without limitation, this could be done by breaking a breakable portion between adjacent branches or by deformation of an arranged material

between adjacent branches.

  In a first aspect of the invention, the internal chamber 17 occupies the majority of the volume of the entire cage 10. With particular reference to FIG. 2a, the external surface 11 defines a diameter 63, excluding the thread 12. The internal chamber 17 has a diameter 64 at least, that is to say a diameter 63

  minus twice the thickness 62 of the branch.

  In a preferred embodiment, the thickness 62 of the branch is chosen so that the volume of the internal chamber 17, including in particular the voids created by the growth windows 16 and 19 and the various channels, occupies the majority of the volume of the entire cage 10. Thus, the present invention provides space for a large volume of material promoting bone growth to be inserted

  inside the device to promote bone growth inside.

  Near the extension end 18, as shown in FIG. 2a with respect to the branches 24, 40 and 26, each of the branches comprises inclined surfaces 28, 34 and 30 respectively. Near the inner termination of each of the inclined surfaces 28, 34 and 30 is formed an inner shoulder 36, 42 and 38, respectively. Referring to Figure 3, the branch

  41 includes an inclined surface 35 and a similar inner shoulder 43.

  As shown in Figure 2a, curved interior surfaces 28 and are inclined to each other at an angle 32. In a preferred embodiment, this angle is approximately 96, although it is understood that a variety of angles can be used depending on the desired extension value, and the distance that an extension element will need to travel to create

extension.

  Referring now to Figure 2b, the cage 10 is shown including a wedge extension member 50 arranged near the end

  extensible 18. The expansion wedge 50 is further illustrated in FIGS. 4 and 5.

  The expansion corner 50 includes first and second opposite corner slopes 52 and 54, which have a beveled curved surface which matches and corresponds to the inclined surfaces 28 and 30 of the legs 24 and 26, respectively. The corner 50 comprises side walls 58 and 60 having substantially planar surfaces capable of engaging with inclined surfaces 34 and 35 of the fixed branches 40 and 41, respectively. In addition, the corner 50 includes a central opening 56 which is capable of receiving a drive tool. In a preferred embodiment, the central opening 56 is threaded to receive one end of a tool

  matching threaded drive.

  It will be understood that, when the corner 50 advances towards the fixed end, the inclined surfaces 52 and 54 act on the inclined surfaces 28 and 30, respectively, to push the branches 24 and 26 away from one of the other at the extendable end 18. When the expansion wedge 50 is inserted along the inclined surfaces 28 and 30, the legs 24 and 26 will tend to extend at the extendable end 18 when 'a deformation appears in the vicinity of the fixed end 20. A substantially continuous connection material 44 connects all the branches and does not allow their extension at the fixed end 20. When the corner 50 advances further towards the fixed end 20, the corner is pushed beyond the shoulders 36 and 38, so that said corner 50 is trapped inside the cage 10. As a result of the engagement of the rear surface 55 of the corner against the shoulders 36 and 38, the coi Extension n 50 is prevented from being driven from the cage, and the cage is maintained in an expanded state with the wedge firmly held in position. It will be understood that the engagement of the flat surfaces 58 and 60 on fixed branches 40 and 41, respectively, tends to slightly expand these branches in a deformable manner, if at all, and therefore they remain substantially fixed in their original position. In addition, the fixed branches 40 and 41 all include shoulders 42 and 43 which engage with the rear surface 55 of the corner 50 as soon as they have passed beyond the shoulders. Thus, the fixed branches 40 and 41 also tend to maintain

  the corner in place and to prevent its expulsion from the internal chamber 17.

  FIGS. 8 to 10 show a variant of the embodiment of the extendable cage of the present invention. In this embodiment, the extendable cage 310 has an outer surface 311 and an outer thread 312 (not shown in Figure 10). As for the embodiment of FIG. 1, the extendable cage 310 comprises two opposite extensible branches 324 and 326, and two opposite fixed branches 340 and 341 connected at the level of the fixed end 320. Each of these branches defines inclined surfaces and interior shoulders near the extendable end 318 adapted to engage with the expansion corner 350 and to receive it. Unlike the embodiment of FIG. 1, the extensible branch 324 comprises bone growth windows 316 and 317, separated by a rib 330. Similarly, the extensible branch 326 comprises two bone growth windows 322 and 323 separated by a rib 332. The use of a plurality of bone growth windows in the extensible branches increases the overall resistance of the branch which may be necessary for longer cages or cages made from relatively weak materials. While this embodiment has been shown with two windows for each extendable branch, it is understood that it can be

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  used more than two bone growth windows without departing from the mind and

part of the present invention.

  FIG. 6 shows an insertion tool 70 suitable for use with an extendable cage according to the present invention. The insertion tool 70 comprises an outer sleeve 70 comprising a drive projection 73 capable of engaging with a drive groove 46 of the extendable cage 10. While the insertion tool 70 is illustrated with a single projection 73, it will be understood that the device comprises an opposite projection (not shown) for coupling with a drive groove (not shown) arranged on the side opposite to the drive groove 46 on the cage 10. The insertion tool 70 further comprises a gripping member 71 capable of transmitting a rotational force to the outer sleeve 72 to insert the cage 10 in a rotary manner. The outer sleeve 72 includes an internal chamber which is occupied by the inner axis 75 of the insertion tool. At the distal end 77, the internal axis 75 of the insertion tool comprises an externally threaded zone capable of engaging with the central opening 56, correspondingly threaded on the inside, from the corner extension 50. At the proximal end of the internal axis 75, there is a gripping member 76 to provide a rotational force to the internal axis 75. A series of external threads 78 are formed on the internal axis 75 near the gripping member 76. A threaded nut 79 is placed on the internal axis 75 and is able to cooperate with the thread 78 to move

  the inner axis 75 relative to the outer sleeve 72.

  In operation, the insertion tool 70 is engaged with the extendable cage as shown in Figure 7a. The drive projection 73 of the outer tube 72 engages in the drive groove 46 of the extendable cage 10 and the threaded distal end 77 of the inner shaft 75 engages by screwing in the threaded opening 56 of the wedge 50. In this way, the wedge 50 is held firmly in position near the extension end 18 while the threaded cylindrical cage is inserted into the intervertebral space. In a preferred embodiment, this device is used on the posterior side of the spine with the element

  extension 50 positioned at the front end of the device.

  Referring to Figure 7a, the expandable cage 10 is threaded into the intervertebral space 92, the thread 12 engaging with the vertebrae 80 and 82 to advance the cage into the disc space and hold it firmly in position

  as soon as it has reached a final position as shown in FIG. 7a.

  As the expandable cage is preferably a cylinder having a uniform diameter, it can be inserted by an insertion tube having a diameter substantially equal to the thread diameter of the cage 10. As shown in FIG. 7a, the surface 88 of the vertebra 80 is in contact with the external surface 11 of the cage 10. It will be understood that in many applications, a portion of the vertebral body will have been removed before the insertion of the

  cage so that the cage 10 engages with the cross-linked tissue bone of the vertebra.

  Similarly, the surface 90 of the vertebra 82 is in contact with the external surface 11 of the cage 10. In its initial insertion position, the alignment 84 of the vertebra 80 and the alignment 86 of the vertebra 82 are in substantially parallel alignment with the longitudinal axis 13 and the extendable arms 24 and 26 of the cage 10. Now referring to FIG. 7b, the threaded end 77 of the insertion device being firmly engaged in the threaded opening 56 of the expansion wedge 50 and the drive projection 73 being engaged in the drive groove 46, the threaded nut 79 is rotated around the external thread 78 to pull the axis 75 inside the outer tube 72 (Figure 6), thereby advancing the expansion wedge 50 towards the fixed end 20. When the expansion wedge 50 advances towards the fixed end 20, the inclined surfaces of the expansion wedge 50 force stretch branches 24 e t 26 to move away from each other in the vicinity of the extendable end 18. Similarly, the vertebrae 80 and 82 are forced to move away from each other in the vicinity of the end extensible 18 so that the alignments 84 and 86 remain substantially parallel to the extensible branches 24 and 26, respectively, and not with the longitudinal axis 13 of the cage 10. In this way, the lordotic curvature of the spine can be established and maintained during the healing process. In addition, the engagement of the expansion wedge 50 with the shoulders described above of each of the branches prevents this expansion wedge from being driven from the cage 10. The insertion tool can be removed and the area unobstructed. inside the internal chamber 17 can be filled with bone growth material to promote bone growth through the device Such bone growth material can include autograft, allograft, bone morphogenic proteins

  in a carrier, or other known materials promoting bone growth.

  The insertion and extension of the variant embodiment shown in the

  Figures 8 to 10 are produced in a substantially identical manner.

  Figures 11 to 13 show another embodiment of an extendable cage according to the present invention. As in the embodiments of the invention described above, the cage 110 is a substantially cylindrical device comprising an external surface 111 defining an external thread 112. The cage 110 defines a substantially cylindrical internal chamber 117. The cage 110 includes a pair of Extensible and opposite branches 152 and 154 separated by a pair of opposite fixed branches 148 and 150. The opposite branch 148 is separated from the extensible branches 152 and 154 by channels 144 and 146, respectively. The fixed branch 150 is separated from the extendable branches 154 and 152 by the channel 147 and a similar channel (not shown), respectively. The cage 110 further includes bone growth windows 129 and 120 formed in the extendable arm 152, and an identical pair of bone growth windows 119 and 121 formed in the extendable arm 154. Each of the windows

  bone growth extends from the outer surface 111 to the inner chamber 117.

  As with the embodiments described above, the cage 110 includes a large unobstructed internal chamber 117 extending along the longitudinal axis 113 from the vicinity of the extendable end 118 towards the fixed end 114. In the embodiment shown in FIGS. 11 to 13, the windows 120 and 121 each include a notch 122 and 123 near

  the extendable end 118, respectively.

  An expander 130 is dimensioned to be received inside the internal chamber 117. The expander 130 includes a first portion 125 having a projection 126 which extends into the notch 122, and a second opposite portion 127 comprising a projection 128 which extends into the notch 123. The projections 126 and 128 operate in conjunction with a threaded plug 124 to hold the stent in position inside the cage 110. The stent 130 further includes an elbow 132. While an elbow can be used in the preferred embodiment, it will be appreciated that the expander 130 may include a fold or hinge between the portions 125 and 127, which allows adaptation to the reduced size configuration shown in Figure 12. The first portion 125 includes a longitudinal axis 72 and a second portion 127 includes a longitudinal axis 70. In the reduced-size insertion configuration shown in Figure 12, the longitudinal axis 70 forms an angle 162 with respect to the longitudinal axis 72. In a preferred embodiment, the angle 162 is approximately 90, although other angles are provided. In an extended configuration (not shown), the angle between the longitudinal axes 70 and 72 can approach 180, an angle of 180 giving the maximum extension of the device The internal chamber 117 is defined by the four branches described above 148, 150 , 152 and 154, each defining a thread portion 160 (shown only partially in FIG. 12). The socket 124 includes a corresponding external thread capable of engaging with the thread 160. In a preferred embodiment, a connection portion 149 extends between the fixed branches 148 and 150 to limit the flaring of the fixed branches when the threaded plug 124 advances towards the extendable end 118. The threaded plug 124 further includes a central opening 136 capable of engaging with an extension of the insertion tool. In a preferred embodiment, the central opening 136 is formed in a hexagonal pattern to accept an insertion tool of similar hexagonal shape (not shown). The cage 110 further includes a drive groove 142 adjacent to the fixed end 114, capable of engaging with an insertion tool projection to allow insertion of the cage between two adjacent bone structures. The drive tool of FIG. 6 can be used with the cage 110 except that the internal axis 75 of the drive tool includes a portion of hexagonal shape at the distal end 77. It will be understood that , when the threaded plug 124 advances by screwing in the direction of the extendable end 118, it pushes the expander 130 until it comes into an expanded state, thus forcing the branches 152 and 154 to move apart. As shown in Figure 13, the expansion mechanism of the present invention provides a large internal cavity to receive the material promoting bone growth. Another embodiment according to the present invention is shown in Figure 14. The cage 180 is shaped similarly to the cage except that it includes a plurality of smaller bone growth windows rather than two large windows in the branches extensible 192 and 194. Variations in the number, size, shape and location of the bone growth windows depending on the desired characteristics of bone growth and the properties of the cage depending on the material are provided herein. invention. In addition, the extension mechanism differs in that the expander 184 is a substantially flat device, that is to say without any elbow or any hinge, having a longitudinal axis 198. The expander 184 is substantially flat and has a first end 188 engaged in the corner 186 formed between the extendable branch 194 and the end wall 199. The opposite end 190 engages with the interior surface 193 of the extendable branch 192, and moves along the latter . The threaded plug 182 engages by screwing with the internal thread 196 formed along the interior surfaces of the branches. It will be understood that, in the unexpanded state, the axis 198 is inclined relative to the longitudinal axis 197 of the cage 180. However, when the threaded plug 182 advances towards the extendable end 181, l the stent 184 moves in the direction of a vertical position relative to the axis 198 moving in the direction of an arrangement perpendicular to the axis 197. The movement of the stent 184 in the direction of a vertical position expands the cage 180. In the expanded position, there is a large unobstructed internal chamber 189 extending from the plug 182 to the opening 187 adjacent to the fixed end 195. Thus, the large internal chamber 189 can be filled with material bone growth to promote fusion

between adjacent vertebrae.

  Referring now to Figure 15, there is shown yet another embodiment of the present invention. The cage 210 includes an outer surface 211 on which a thread 212 is produced. The cage 210 includes a groove

  246 drive capable of receiving a drive tool like that described above

  before. The cage 210 further includes a plurality of windows 214 providing communication between the external surface 211 and the internal chamber 217. The internal chamber 217 is delimited by inclined surfaces 216 and 215 (shown in dotted lines), tilting from the extendable end 222 towards the fixed end 224. The slope of the inclined surfaces 216 and 215 could also be reversed to allow extension by the movement of the plug 218 in an opposite direction. The plug 218 includes an external thread capable of engaging with the thread 220 of the surfaces 216 and 215. It will be understood that, the plug 218 advances by screwing in the direction of the fixed end 224, the branches 230 and 231 are expanded moving away from each other. As shown in Figure 15, the branch 230 includes a width zone

  reduced 225, able to deform when the gripping element 218 advances.

  As shown, the grip member 218 includes a central opening 237 adapted to receive an insertion tool extension to provide rotational force. In addition, while the drive groove 246 is shown as being formed on the extendable end 222, it will be understood that, for certain insertion techniques, it will be desirable to have an insertion tool groove 246 formed on the fixed end 224. In addition, a central opening can be made in the fixed end 224 for the passage of an extension of insertion tool for

engagement with plug 218.

  While the plug 124 of the embodiment of Figure 11, the plug 182 of the embodiment of Figure 14 and the plug 218 of the embodiment of Figure 15 have been shown and described as having an external thread for the engagement with a corresponding thread defined on the branches of the device, it will be understood that all the branches, or only the fixed branches of each of the devices, can be formed to define a series of pawls. With a pawl structure, each of the plugs 124, 182 and 218 can be defined as having an outer surface capable of advancing over the pawls to extend the device while having a rear portion capable of engaging with the pawls to prevent the 'expulsion. In this way, the plugs can advance without screwing. When using this technique, the insertion tool may be able to firmly hold the outer cage to prevent it from advancing further as a result of the pushing or pulling force exerted on the plug members. In addition to the modifications to the plug, the cage itself can be configured for push-in insertion and

  may have shapes other than cylindrical.

  The cages according to the present invention are preferably formed from a biocompatible material having sufficient strength to support the loads which will be placed on them for a given application. In addition, in preferred embodiments, the material should have sufficient flexibility to withstand at least a small amount of deformation as a result of the extension process. Alternatively, for some devices, it may be desirable to provide hinge points rather than allowing the material to withstand deformation. Preferably, the material used to form the cages of the present invention is a medical grade titanium alloy. In addition, the devices could be made of stainless steel, various types of plastics, various composites including carbon fiber devices, bone or

bone substitutes.

  While the invention has been illustrated and described in detail in the drawings and

  in the preceding description, this should be considered as being given to

  by way of illustration and not as being limiting, it being understood that only the preferred embodiment has been shown and described and that it is desired to protect all the changes and all the modifications which come within the scope of the invention.

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Claims (19)

  1. An extensible fusion cage (10, 10 ...) able to be inserted between a pair of vertebral bodies (80, 82), comprising: a body comprising an external surface (11) engaging with the bone and a inner surface (21) defining an internal chamber (17 ...) and a retaining mechanism (36, 38; 160 ...), said body defining at least a first branch (24, 26), said branch comprising a portion fixed (20) connected to said body and an extendable portion (18) movable relative to said body; and an extension member (50;  350 ...) sized to be at least partially received inside said internal chamber and to engage with said extendable portion (18), said extension element comprising an external surface (52, 54) suitable for s engaging with said retaining mechanism, so that the movement of said extension element inside said internal chamber pushes said extendable portion of the first branch (24, 26) to move it relative to said body, extending by consequently said body, said extension element being held in position by engagement with said body restraint.   2. The cage of claim 1, wherein said surface engaging with   the bone is threaded on the outside (12).   3. The cage of claim 1, wherein said retaining mechanism is a thread (160) defined on said interior surface and said extension member   includes a threaded plug (124) and configured to engage with said thread.   4. The cage of claim 1, wherein said retaining mechanism is an inner shoulder (36, 38) and said extension member is a wedge (50) adapted   to engage with said shoulder.   5. The cage of claim 1, wherein said interior surface defines an inclined surface (215, 216) threaded and said extension member (218) comprises   a corresponding external thread.   6. The cage of claim 1, wherein said body defines a second branch (26) opposite to said first branch (24), said second branch 21 2782632   having a fixed portion (20) connected to said body and an expandable portion, such that the movement of said extension member (50) inside said internal chamber (17) pushes both said first (24) and second   branches (26) to separate them from each other.   7. Extensible cage (10; 310) capable of being inserted between a pair of vertebral bodies (80, 82), comprising: a tubular body having an outer surface (11) engaging with the bone and an inner surface 21) defining an internal chamber (17), an inclined surface (28, 24, 30) and a retaining mechanism (36, 38), said body defining a fixed portion (20) and a movable portion (18), said inclined surface being arranged near said movable portion; and an extension member (50; 350) having an outer surface configured to engage with said inclined surface (28, 30) and said retaining portion, so that said extension member is moved against said inclined surface for moving said movable portion to extend said body and said extension member is held in position by engagement with said retaining mechanism.   8. The cage of claim 7, wherein said retaining mechanism includes at least one ledge defined near said inclined surface (28, 30) and said extension member (50; 350) includes a shoulder which cooperates with said   rim to maintain the position of said extension element.   9. The cage of claim 7, wherein said retainer is a thread (160; 215, 216) and said extension member (124; 218; 182) includes a corresponding external thread.   10. The cage of claim 7, wherein said surface engaging with   the bone is an external thread (12; 212; 312).   11. The cage of claim 7, wherein said body includes a fixed end (20) and an extendable end (18 ...) and further includes four longitudinal channels (14, 22) extending between said outer surface (11 ) and said inner surface (21) and from said extendable end (18) to said fixed end (20), said channels defining first and second branches 22 2782632   (24, 26) opposite separated by third and fourth opposite branches (40, 41). 12. The cage of claim 11, wherein said extension member (50; 124, 182 ...) extends said first and second opposite legs (24, 226 ...) at a speed substantially greater than said third and fourth opposite branches (40, 41 ...).   13. The cage of claim 7, wherein said body further defines openings (16, 19; 319, 317 ...) for bone growth between said surface   interior (21) and said exterior surface (41).   14. The cage of claim 7, wherein said internal chamber (17; 117 ...) occupies at least 50% of the volume of the cage, thus allowing the insertion of a large amount of material promoting bone growth at l inside the internal chamber.   15. The cage of claim 7, wherein said extension member defines a central opening (56; 136 ...) communicating with said internal chamber (17, 117 ...).   16. An interbody fusion apparatus, comprising: a hollow body (10; 110, 310 ...) having an outer surface (11 ...) engaging with the bone and an inner surface (21 ...) defining an internal chamber (17 ...) and a retaining mechanism (36, 38; 160, 196 ...), said body comprising a fixed portion (20) and an expandable portion (18, 118 ...) and defining at least a first branch (24) and a second branch (40 ...), said first branch and said second branch being interconnected at said fixed portion, and said first branch being movable relative to said second branch at level said expandable portion; an extension element (50; 350;  124 ...) dimensioned to be at least partially received inside said internal chamber (17 ...) and comprising an external surface capable of engaging with said retaining mechanism (36, 38 ...), so that the movement of said extension member inside said internal chamber pushes said first branch to move it relative to said second branch, thereby extending said portion 23 2782632   extendable, said extendable member being held in position by engagement with said body retaining mechanism; and an insertion tool (70) comprising an external sleeve (72) capable of engaging with said body to transmit a compressive force and an internal axis (75) movably arranged inside said external sleeve and dimensioned to be inserted into at least a portion of said internal chamber, said internal axis having a proximal end and a distal end (77), said distal end being configured to retractably engage with said extension member (50; 350 ...), so that the movement of said internal axis relative to said external sleeve displaces said element   of extension with respect to said body.   17. The apparatus of claim 16, wherein said extension member (50; 350 ...) includes a central opening (56) having a thread and said distal end (77) of said inner axis (75) includes an outer thread corresponding.   18. The apparatus of claim 16, wherein said extension member (124) includes a hexagonal central opening (136) and said distal end   of said inner axis has a corresponding outer hexagonal shape.   19. The apparatus of claim 16, wherein said body further defines a drive groove and said outer sleeve (72) defines a projection   drive (73) coupling in correspondence.